Online: GPS-GAP

GPS has demonstrated a stellar performance ever since its inception. In fact the satellites typically operate beyond their expected lifetime which potentially creates obstacles to the timely modernization of the system. GPSGAP (GPS, Geodesy and Application Program) is an online educational initiative by the University of Maine that offers in-depth knowledge about this fantastic system and its uses.

My enthusiasm for GPS began when testing the experimental Macrometer receiver during the summer of 1982 at M.I.T. over a 30 km baseline from Woburn, MA, to Mount Watchusett. The satellite visibility ranged from about 6 p.m. to midnight in New England. Many of the sunset watchers at the summit were puzzled by my activities and impressed by the huge piece of equipment in the back of my station wagon, the abundance of cables, and the strange looking antenna (so they thought). Their puzzlement about what I was up to was refl ected in some of their comments, such as“Is this thing taking off?”, or “Are you on our side?” Of course, there was plenty of time until midnight to be entertained by Fourier transforms and such on the computer screen, and to ponder the unlimited potential of GPS. Whatever has evolved since those days in terms of civil uses of GPS needs no further explanation.

Those long evening hours on top of Mount Watchusett allowed not only double-checking the fantastic repeatability of the observed baseline vector night after night, but also to marvel at the science behind all of that. There was the prospect that GPS could revolutionize my fi eld of specialization, i.e. geodesy and that we could gain a better understanding of the variations of the atmosphere. There certainly was curiosity as to what signals the satellites actually transmitted. I was told that it was so weak that it was below the background noise. And yes, why was this antenna so large that onlookers thought it might take off and why was the computer crunching all night? What precisely were those carrier phases we used to compute the baseline? How did observations from a global network of tracking stations arrive at the control center? The latter certainly caught my attention since I was using the forerunner of the internet, the BITNET, to supervise a graduate student in Maine, thus gathering my fi rst experience with distance education. Of course, during the day we talked about gravity, solar radiation pressure, relativity, multipath, ambiguity fi xing, modeling, and so on.

My amazement with the science underlying GPS satellite surveying made me rush to establish a graduate course in GPS that Fall at the University of Maine. The urge to tell the GPS story propagated into three editions of my book GPS Satellite Surveying and into the series of GPS-GAP internet courses. As a faculty member I have been wondering for a long time when
should we stop recommending astronomy to our students to fulfi ll science requirements and alternatively recommend GPS, which is “much closer to home” and has such an abundance of science to offer. Of course we no longer need the station wagon and midnight observations.

GPS-GAP has been designed as a cost effective approach to education that takes advantage of the internet.

The courses are offered asynchronously, the class size is one, i.e. there is individualized instruction, and a course can start any. The time constraints of the traditional semester calendar do not apply. The courses can be taken in the work place, at times convenient for the student, and at a pace that fi ts the needs of the individual. A dedicated server runs live computations with relevant data and actual GPS observations. To avoid incompatibilities in programming skills, all computations are programmed with Mathcad, which can be learned “on the fl y” because of its intuitive graphical programming interface. Students are not required to buy any software. An internet browser is all that is necessary to take the courses. Computer graded exams are available for assessment.

Details about GPS-GAP are found at www.gnss.umaine.edu. The material covered is closely tied to the textbook GPS Satellite Surveying (3rd edition). The quiz questions play an important and integral part in the iterative learning strategy. The courses have been designed as 1-credit hour units, allowing the students to navigate the sequence of the courses and taking advantage of their prior knowledge of the subject.

Interested students might consult the textbook to get an even better feeling about the depth of coverage. The material is presented with suffi cient depth as needed for understanding all geospatial positioning accuracy levels, ranging from 100 meter to millimeter, in either real-time or post process. Such topics as ambiguity fi xing, conventional and network RTK, VRS (Virtual Reference Station) networks, geometryfree solutions and precise point positioning of course are included.